A unique fossil record from neptunian sills: the world’s most extreme example of stratigraphic condensation (Jurassic, western Sicily)

Abstract

Neptunian sills at Rocca Busambra, a fragment of the Trapanese/Saccense Domain in western Sicily, host the
most abundant ammonite and gastropod fauna which has ever been recorded from the Jurassic of the western
Tethys. The fauna is dominated by parautochthonous organisms which were swept into the sills by gentle
transport. Ammonites are characterized by perfect preservation and small size, a feature which is due to the
predominance of microconchs but also of stunting. The most complete sill is 0.7 m thick and could be separated
into 17 levels which range in age from the early Toarcian into the late Kimmeridgian, thus representing the most
extreme case of palaeontologically and depositionally documented stratigraphic condensation in Earth history.
The unique feature of the Rocca Busambra sills is due to the interaction of three processes: extreme stratigraphic
condensation on the sea floor, weak tectonic fracturing of the host rock and repeated reopening on top of already
existing sills. Contrasting percentages of gastropods in individual levels reflect sea-level oscillations which
correspond to long known low- and highstands during the Jurassic of the western Tethys. Comparisons with
other ammonite-bearing sill faunas reveal several similarities, but represent only short-timed phases of tectonic
pulses and deposition.